EXFO FTB-800 Series for FTB-1 - Manual (Page 266)

Test Results
256 FTB-700G/800 Series
Alarms/Errors
E4
Alarms
Note: Only AIS is available when the framing is set to Unframed.

AIS (Alarm Indication Signal)
RX: An unframed all-ones signal is received.

LOF (Loss Of Frame)
RX: Four consecutive incorrect frame alignment signals is received.

RAI (Remote Alarm Indication)
RX: Bit 13 of a framed E4 is set to “1”.
Errors

FAS (Frame Alignment Signal)
RX: Bits 1 to 12 of the first frame differ from 111110100000.